1. Field of the Invention
The invention pertains generally to electrothermal actuator systems and is particularly directed to positive temperature coefficient heating elements for such systems.
2. Prior Art
Actuators are presently being accepted to accomplish many tasks formerly assigned to electric and vacuum motors, solenoids, and cables. This is especially true in difficult areas such as remotely operating baffles, dampers, or latch mechanisms. The actuator's cost, size, and weight advantages over generally available conventional systems are indicators that this trend will continue.
A particularly favorable actuator is of a type termed electrothermal. In this mechanism a heater assembly is provided to convert electrical energy into thermal energy. The thermal energy is employed to expand a medium which applies force to an operating member, usually a piston. The piston can be used to cause actuations as the medium is expanded throughout the operation.
The thermally expansive medium usually is heated through a phase change, either solid to liquid or liquid to gaseous, causing a resulting increase in volume to produce the maximum amount of pressure for a given amount of energy.
In the prior art a number of different heaters for the thermally expansive mediums have been utilized. In some cases fixed resistance heaters have been looked to for supplying thermal energy in these actuator systems. However, fixed resistance heating elements have been difficult to maintain at a constant temperature and require some form of external thermostatic control to provide suitable actuation characteristics.
To solve this regulation problem common to fixed resistance heaters some in the art have turned to positive temperature coefficient (PTC) heaters. A PTC material is one which exhibits a low resistance at ambient temperatures but when such a material is raised above an anomaly or Curie temperature it exhibits a rapid increase in resistance of at least several orders of magnitude. This is an ideal characteristic for an actuator heater; whereby the heater can draw large amounts of current and input power (inrush) to reach the Curie temperature thus expanding the working medium and then subsequently cut off its power by increasing the resistance. Thereafter, as the temperature changes the resistance will adjust to allow more or less current to be drawn and consequently readjust the temperature back to the operating point. These devices are therefore essentially self-regulating to a considerable extent.
Examples of linear actuators having PTC heating elements are found in a U.S. Pat. No. 3,686,857 issued to Berg and a U.S. Pat. No. 3,782,121 issued to Marcoux et al. The patents illustrate a PTC heater with a disc shape for energizing a linear actuator including a working medium that changes from a solid phase to a liquid phase.
Configurations other than disc shaped for PTC heaters are known but exist in commercial products dissimilar to linear actuators. U.S. Pat. No. 3,632,971 issued to Flanagan describes an elongated annulus of PTC material in a heater element for a consumer product. This heater generally lacks at least some of the important characteristics necessary for heaters used in fast acting linear actuators.
Normally actuators using PTC disc heaters work efficiently in the environment in which they were intended to operate but there are certain situations where more rapid actuations are needed. To accomplish rapid actuation the heater must be configured to thermally expand the working medium as quickly as possible. Conventional PTC disc actuators have not been presently able to meet the response times desired by designers.
Thus attempts have been made to design fast acting actuators by forming arrays of PTC discs. These arrays, however, are complicated in construction and electrode structure and are relatively expensive. Moreover, because of their complicated construction requiring multiple heater mountings they are more susceptible to failure from the physical shocks which are encountered in many actuator environments.